I want to use the various Libre Office products as my defaults. XUbuntu wants me to use something else. I win.

Open up the file manager (<Super>+F by default).

Browse to the file you want to open. (Use up/down/left/right arrows to navigate through the folder. Use Alt+Up to move up a folder; Alt+Down to move back down into a folder you've already visited. You *are* using keyboard shortcuts, right?!?!)

When you've got the desired file highlighted, open up the context menu (FN+Print Screen on my K800) and choose "Properties...".

TAB to the "Open with..." dropdown, use the arrow keys to select the desired default.

Close

Now XUbuntu will open up files of the same extension with the selected program by default. So if you're, say, downloading from the web and you choose to open rather than save, you'll be in the right program. Or, if you're browsing your files and you hit [enter] to open up a file, still the right program.